Function LCS(a As String, b As String) As String
    If Len(a) = 0 Or Len(b) = 0 Then Return ""
    While Len(b)
        For j As Integer = Len(b) To 1 Step -1
            If Instr(a, Left(b, j)) Then Return Left(b, j)
        Next j
        b = Mid(b, 2)
    Wend
End Function

Print LCS("thisisatest", "testing123testing")
Sleep